n.
1. The process of Windows rotting or the condition of being rotten: “The winrot spread quickly, rendering the system slow and unstable”.
2. Any of several Windows programs characterized by the breakdown of stability, speed or security. “Don't install Norton. It is absolute winrot”.

My nephew only used my computer for 10 minutes, but once he was done the winrot was so bad I had to reinstall Windows.

This is what inevitably happens to every computer running the MicrosoftWindows family of operating systems. Once your computer has been running for a while, the registry and filesystem gets so full of useless junk from old programs and bloatware, that it slows the whole system down. After you've defragged a million times and cleaned up everything imaginable, you still can't get your computer back to the way it was when you first installed Windows. The only cure is to reinstall a fresh copy of the operating system. That will keep you going for at least another 3 months.